There are numerous references within the prior art for the removal of dielectric films, silicon dioxide, silicon nitride by Chemical-Mechanical Polishing (CMP) regulated and moderated by the interaction of abrasive particulate within a slurry. Changes in the slurry solution properties have a profound effect on the polishing chemistry and relative removal rates of dielectric films. Common slurry solution properties include pH, temperature, slurry particle composition, its size and shape, degree of agglomeration, and slurry weight percent. While the slurry vendor has control over most slurry properties, the pH and temperature can be controlled during the polishing process and can have a strong influence on CMP results.
CMP relies on mechanical means with chemical activity to remove and ultimately planarize the top film or films on wafers during semiconductor processing. The mechanical action during CMP, including table speed, applied force, pad hardness, etc., are typically used to control rate, planarity, and uniformity. The chemical portion of CMP is usually exploited to achieve selectivity and help deal with issues of pattern dependence such as erosion and dishing. Silica slurries are typically prepared with pH greater than 8, while ceria slurries have pH ranging from 4 to 10. In both cases the pH of the suspension can have a strong effect on polish rate and selectivity. Knowing the effect of pH on rate and film selectivity can be used to help understand and control the CMP process and achieve the desired result.
A study of the pertinent prior art references indicates that there are a few compositions of CMP slurries which provide a fairly high oxide to nitride removal rate selectivity. Those that are known are summarized below, but none of these prior art references have the composition or the wide working range of pH.
Conventional CMP slurries used for polishing oxides typically comprise of an abrasive in alkaline medium (high pH). They polish both oxide and nitride at substantial rates with a selectivity of about 10 or below. Hence new slurries which can provide a very high oxide to nitride selectivity are needed to overcome the problem of polishing nitride and the underlying silicon substrate.
There are a number of prior art patent references for slurries used in CMP: (1) U.S. Pat. No. 5,738,800 issued in April 1998 to Hosali, et al.; (2) U.S. Pat. No. 5,759,917 issued in June 1998 to Grover et al.; (3) EP 0 786 504 A2 published in July 1997 by Kodama et al.; (4) EP 0 846 740 A1 published in June 1998 by Ronay; and (5) EP 0 853 335 A2 published in July 1998 by Morrison et al.
Additional prior art references in the form of technical publications that are related to the present invention are summarized below.
Chemical Mechanical Planarization of Microelectronic Materials; “8.1.2 Shallow Trench Isolation”, by J. M. Steigerwald, S. P. Muraka, and R. J. Gutman; ISBN 0-471-13827-4 (Jon Wiley & Son, Inc.1997), pages 273–274. This reference gives a general discussion of the desirability of using Shallow Trench Isolation in place of Local Oxidation of Silicon (LOCOS). While this reference teaches that conventional CMP processes have low selectivity between oxides and nitrides, no insight into a potential correction for this problem is taught.
A High Oxide:Nitride Selectivity CMP Slurry for Shallow Trench Isolation; by Sharath Hosali and Ray Lavoie; in Electrochemical Society Proceedings, Volume 98-7, pages 218–234. This reference teaches a slurry that enhances the selectivity rate between silicon oxide and silicon nitride. The slurry taught has cerium oxide as an abrasive with a proprietary solution that inhibits the removal rate of the silicon nitride. The results of this prior art references indicate a high rate of selectivity for unpatterned wafers, however, the selectivity for patterned wafers is almost the same as with the conventional slurry that it was compared against.
Application Of Ceria-Based High Selectivity Slurry to STI CMP For Sub 0.18 μm CMOS Technologies; by Ki-Sik Choi, Sang-Ick Lee, Chang-Il Kim, Chul-Woo Nam, Sam-Dong Kim, and Chung-Tae Kim; CMP-MIC Conference, Feb. 11–12, 1999. This paper teaches the use of a ceria based slurry for CMP purposes in a STI setting. While, the slurry discussed may have been selective, there is no teaching related to the manufacturing of the slurry. Moreover, in order to prevent dishing from the CMP process, dummy patterns are required. Also, there are some problems related to scratches from the ceria abrasive which require filtering to correct.
A Production-Proven Shallow Trench Isolation (STI) Solution Using Novel CMP Concepts; by Raymond R. Jin, Jeffery David, Bob Abbassi, Tom Osterheld, and Fritz Redeker; in CMP-MIC Conference on Feb. 11–12, 1999. This reference discusses the previously discussed problem of having to use dummy patterns to reduce dishing. The solution offered by this prior art reference is to employ a low selectivity, or no selectivity, slurry for minimizing dishing during over polishing in combination with special system apparatus and polishing heads.
A Wide Margin CMP And Clean Process For Shallow Trench Isolation Applications; by Brad Withers, Eugene Zhao, Rahul Jairath; in CMP-MIC Conference on Feb. 11–12, 1998, pages 319–327. This reference addresses the problems of process cost and complexity due to the need for block masks, pattern resist etch, high selectivity material overlayers or implementation of dummy active areas.
It should be readily apparent from the foregoing discussion that there remains a need within the art for a method of chemical mechanical polishing and a slurry for that purpose that is selective and has the wide working range of pH of the present invention.